Heat detector with a limited angle of vision

ABSTRACT

An uncooled thermal matrix detector having a given spectral sensitivity range and being formed from pixels which are thermally insulated from each other, each pixel including an absorbing element which is thermosensitive in the spectral sensitivity range. The thermal matrix detector also includes on its face, turned toward the incident radiation to be detected, of each thermosensitive absorbing element, a biperiodic grating of elementary blocks limiting the viewing angle of the detector, the pitch of the grating being less than the mean wavelength of the spectral sensitivity range. The thermal matrix detector can be applied to so-called BLIP uncooled thermal detectors.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to the field of thermal detectors, especiallyso-called BLIP (background limited performance) high performance thermaldetectors. These thermal detectors have a sensitivity which istheoretically limited by the thermal conductivity noise between thedetector and the external environment. This noise arises from thefluctuation in the number of photons coming from the externalenvironment and arriving on the surface of the thermal detector.

2. Discussion of the Background

Thermal detectors are detectors which measure, directly or indirectly,the heat produced by the incident radiation to be detected when itarrives on the surface of the detector, which heat is transformed into atemperature rise at the detector surface. Preferably, the spectralsensitivity range of the thermal detectors is situated in the infrared.

Generally, the scene observed by a thermal detector is “seen” by thelatter under a certain viewing angle called viewing angle of the scene.When the thermal detector is part of an optical architecture, theviewing angle of the scene is determined by the aperture of the entranceoptics of the optical architecture. In order to limit the noise receivedby the thermal detector, it is beneficial to limit the viewing angle ofthe thermal detector to a given angle, preferably equal to the viewingangle of the scene. Thus, without loss of signal, the noise received bythe thermal detector will be decreased.

Thermal detectors are subdivided into two families, cooled thermaldetectors and uncooled thermal detectors. A cooled thermal detectorgenerally comprises a cold shield. This cold shield is pierced by anaperture which limits the viewing angle of the detector to the viewingangle of the scene. This is because, the cold shield is at lowtemperature, typically about 77 kelvin. Thus the thermal emission of thecold shield is negligible compared to the thermal emission of theobserved scene since the latter is much hotter, typically 300 kelvin.The cold shield provides a marked improvement in the performance of athermal detector by limiting its viewing angle. Similarly, the apertureof the cold shield may contain a spectral filter cooled like the coldshield. Thus, the spectral sensitivity range of the thermal detector maybe reduced to certain windows corresponding advantageously to thewindows of atmospheric transparency.

Since the uncooled thermal detectors do not have a cold shield, thissolution is difficult to apply without also cooling the detectors, whichwould decrease their advantage. Consequently, the invention isapplicable most particularly to uncooled detectors.

According to the prior art, several types of uncooled thermal detectorshave been proposed. The majority of them are detectors withmicrobolometers, i.e. matrix detectors, the pixels of which are made ofa material whose resistance varies according to the temperature. Thepixels have one or more layers depending on the type of thermaldetector. However, one property common to these different types of pixelis that of having an absorbent layer which absorbs the majority of theincident radiation even when the latter has an almost horizontalincidence, i.e. making a large angle with the normal to the detectorsurface.

SUMMARY OF THE INVENTION

The invention makes it possible, as in the case of cooled thermaldetectors, to limit the viewing angle of the detector, preferably to theviewing angle of the scene. For this, each pixel is individuallyprovided with a biperiodic structure, the pitch of which is smaller thanthe mean wavelength of the spectral sensitivity range of the detectorand which has the effect of limiting the viewing angle of the thermaldetector. This limitation is obtained by the transition of the pixelfrom a rather absorbing state to a rather reflecting state depending onthe angle made with the normal to the surface of the detector by theincident radiation upon its arrival on the surface of the thermaldetector. The choice of parameters for this biperiodic structure makesit possible to limit the viewing angle of the detector to the valuerequired by the particular application envisaged. This biperiodicstructure is a biperiodic grating of elementary blocks, the angularselectivity of which is high enough to allow the usual conditions of theenvisaged applications to be satisfied.

According to the invention, an uncooled thermal matrix detector isprovided having a given spectral sensitivity range and being formed ofpixels which are thermally insulated from each other, each pixelcomprising an absorbent element which is thermosensitive in the spectralsensitivity range, characterized in that on the face, turned toward theincident radiation to be detected, of each thermosensitive absorbingelement is placed a biperiodic grating of elementary blocks limiting theviewing angle of the detector, and in that the pitch of the grating isless than the mean wavelength of the spectral sensitivity range.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be n and other particular features and advantageswill appear with the help of the description below and of the appendeddrawings, given by way of examples, where:

FIG. 1 shows schematically the structure of a particular type of pixelof which the thermal detector according to the invention is formed;

FIG. 2 shows schematically an optical architecture containing a thermaldetector according to the invention;

FIG. 3 shows schematically an example demonstrating the effect of thepitch of the grating on the angular selectivity of the grating in athermal detector according to the invention;

FIG. 4 shows schematically an example demonstrating the spectralselectivity of a thermal detector according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows schematically the structure of one particular type of pixelof which the thermal detector according to the invention is made. Thethermal detector according to the invention is a matrix detector, i.e.it is in the form of a two-dimensional matrix of individual pixels. Oneof these pixels is shown in FIG. 1. The pixel is shown in perspective.The pixel is illuminated by incident radiation coming from the externalenvironment and in particular from the scene to be observed. Thisradiation may be divided, on the one hand, into incident radiation to bedetected which is the useful signal represented by solid arrows and, onthe other hand, into undesirable radiation which is noise. The pixelcomprises an absorbent element 2 which is thermosensitive in thespectral sensitivity range of the thermal detector, i.e. an element ofwhich at least one measurable property varies according to itstemperature. The thermosensitive absorbent element 2 is preferably flat.It is, for example, a square of side p. The thermosensitive absorbingelement 2 may comprise, depending on the technologies used, one or morelayers of material, some of which may then be absorbing, othersthermosensitive, yet others insulating, while some possess many of theseproperties at the same time. Preferably, this thermosensitive absorbingelement 2 comprises a microbolometer, which may or may not be coatedwith an additional absorbing layer, the resistance of the element 2 thenbeing dependent on its temperature.

The pixel also comprises elementary blocks 30. The elementary blocks 30are placed on one 20 of the faces of the thermosensitive absorbingelement 2. The face 20 of the element 2 bearing the elementary blocks 30is the face 20 turned toward the incident radiation to be detected,represented by the solid arrows. The general direction of the incidentradiation to be detected is generally substantially along the normal tothe surface of the thermosensitive absorbing element 2. By choosing themean propagation direction of the incident radiation to be detected asthe high to low direction, the elementary blocks 30 are located abovethe thermosensitive absorbing element 2. The set of these elementaryblocks 30 forms a grating 3, the elementary blocks 30 of which form thepatterns. The grating 3 may, for example, be deposited or even etched onthe thermosensitive absorbing element 2. The grating 3 is biperiodic,i.e. periodic along two directions parallel to the surface of thethermosensitive absorbing element 2. The periodicities along the twoaxes X and Y are advantageously equal. These two directions arepreferably perpendicular, they are shown by the X and Y axes. The meandirection of the incident radiation to be detected is generallyperpendicular to the plane formed by the X and Y axes.

Preferably, the pixel comprises, below the thermosensitive absorbingelement 2 of thickness e₂, a substrate 1 reflecting in the spectralsensitivity range of the thermal detector. The pixel thus comprises anelectrically and thermally insulating layer 4 which is advantageouslyformed by a vacuum or even by air. The substrate 1 is preferably commonto all the pixels, but the pixels remain, even so, thermally insulatedfrom each other by the presence of the electrically and thermallyinsulating layer 4. The pixel preferably comprises electrical connectionleads 5, in general, two or even four as in FIG. 1, which allow thedirect or indirect reading of the property which is variable accordingto the temperature of the thermosensitive absorbing element 2. Thethermal conductivity of these connection leads 5 must be as low aspossible, at least less than a given threshold which depends on theenvisaged application. In the preferred embodiment envisaged below wherethe layer 4 is formed by a vacuum, the thermosensitive absorbing element2 is in the form of a microbridge, with one or more layers, suspendedabove the reflecting substrate 1 using several electrical connectionleads 5.

To facilitate the absorption phenomenon, which occurs in thethermosensitive absorbing element 2, the thickness e₁ of the vacuumlayer 4 is preferably chosen so that, on one hand, an electrical fieldnode is situated at about the reflecting substrate 1 and, on the otherhand, an electrical field antinode is situated in the thickness e₂ ofthe thermosensitive absorbing element 2. The sum of the thickness e₁ ofthe electrically and thermally insulating layer 4, which is a vacuumhere, and of the half thickness e₂/2 of the thermosensitive absorbingelement 2 is almost equal to a quarter of the mean wavelength λ₀ of thespectral sensitivity range of the thermal detector. Thus the absorptioneffect in the thermosensitive absorbing element 2 is maximum.

The biperiodic grating 3 of elementary blocks 30 has a pitch a and alength d of elementary block. The grating 3 of elementary blocks 30 actas a spatially resonating element which limits the viewing angle of thedetector to a value which is advantageously equal to the viewing angleof the scene to be observed. The pitch a of the grating plays animportant role in this property, the pitch a must be less than the meanwavelength λ₀ of the spectral sensitivity range of the detector.Examples of values of the pitch a will be given in FIG. 3. It will benoticed that the smaller the ratio a/λ₀, the smaller the value of theviewing angle of the detector.

The grating 3 has a thickness e₃. This value must be high enough so thatthe grating 3 has this angular selectivity effect. Preferably, thegrating 3 is thick, i.e. the grating 3 has a thickness e₃ which isgreater than a tenth of the mean wavelength λ₀ of the spectralsensitivity range of the detector. Where this thickness e₃ is largeenough, it makes it possible to better control, via the grating 3, themultiple reflections of the incident radiation which are produced atabout the elementary blocks 30 of the grating 3 and which contribute tothe value of the overall reflection coefficient of the detector.

The elementary blocks 30 have, for example, a parallelepipedal shape asin FIG. 1, which has the advantage of simplicity. The elementary blocks30 preferably have a pyramidal shape which further makes it possible topromote absorption by multiple reflections on the elementary blocks 30of the grating 3. The apex of the pyramidal shape is oriented upward,i.e. toward the incident radiation to be detected. The pyramidal shapealso allows better impedance matching between the external environment,generally vacuum or air, and the material forming the thermosensitiveabsorbing element 2. For reasons of polarization symmetry, theelementary blocks 30 have equal dimensions along the X and Y axes, theyare then like squares, when seen from above.

The elementary blocks 30 of the grating 3 are preferably made of anelectrically conducting material. The electrical conductivity of theelementary blocks 30 is intermediate between dielectrics and perfectconductors. The application envisaged will make it possible to determinethe optimum conductivity of the material forming the elementary blocks30. The ratio ε″/ε′ of the imaginary part to the real part of theelectrical permittivity of the material is typically greater than unity.The material is for example of the type such as doped amorphous siliconor even vanadium dioxide. However, other materials used asthermosensitive absorbent in uncooled thermal detectors may be suitable.

FIG. 2 shows schematically a type of optical architecture containing athermal detector 6 according to the invention. The surface 60 is thesensitive surface of the detector 6 on which the pixels previouslydescribed, are placed. The optical architecture also comprises entranceoptics 7 collecting the incident radiation to be detected from a scene 8to be observed. The general direction of the incident radiation to bedetected is represented by an arrow in dotted lines. The entrance optics7 have an aperture defined by their diameter Φ and by its focal lengthf. The viewing angle of the scene is equal to 2 arctan (Φ/2f), which isapproximately equal to Φ/f for relatively small angles. Thus the viewingangle of the detector will advantageously be limited to a value θ almostequal to 2 arctan (Φ/2f).

The size of each pixel is of the order of a few times the maximumwavelength λ_(m) of the spectral sensitivity range of the detector.Thus, in the majority of usual cases, each pixel will have a surfacearea which covers at least the Airy disk for the maximum wavelengthλ_(m) of the spectral sensitivity range of the detector thus leading toa high signal to noise ratio for the thermal detector considered, sincethe Airy disk contains the bulk of the incident radiation energycorresponding to a point of the scene. The diameter of this Airy diskcorresponds to the product of the aperture of the entrance optics 7 andthe maximum wavelength λ_(m), and is thus equal to 2 λ_(m) arctan(Φ/2f).

FIG. 3 shows schematically an example demonstrating the effect of thepitch a of the grating 3 on the angular selectivity of the grating 3 ina thermal detector according to the invention. The example correspondsto a thermal matrix detector, the pixels of which are in keeping withthat shown in FIG. 1. The spectral sensitivity range is band III in theinfrared, the mean wavelength in question being equal to λ₀=10.6 μm. Thethickness of the vacuum layer 4 is equal to e₁=1.45 μm and the thicknessof the thermosensitive absorbing element 2 is equal to e₂=2.3 μm. Here,the material of the element 2 is doped amorphous silicon, the real andimaginary relative permitivities of which are equal to 1 and 1.68respectively. The thickness of the grating 3 made of the same materialas the thermosensitive absorbing element 2, is equal to e₃=2 μm. Theratio d/a for the elementary blocks is taken as equal to 0.9 and theeffect of the various values of pitch a of the grating 3 are studied.The size of each pixel, i.e. the side of the square formed by the pixel,are equal to about p=50 μm. There are therefore about 100 blocks 30 pergrating 3. The following values correspond to an ideal case which doesnot take into consideration the finished nature of the grating 3.

FIG. 3 shows the variation of the overall reflection coefficient R ofthe thermal detector which comprises a large number of pixels, expressedas a percentage %, according to the angle of incidence a of the incidentradiation with respect to the normal to the detector surface, expressedin degrees/°. The curves A, B and C show this variation for values ofpitch a of the grating 3 equal to 4.85 μm, 5 μm and 5.2 μm,respectively. The angular limitation thresholds, which are quite abrupt,are equal to 1.5 degrees, 8.5 degrees and 19 degrees, respectively. Theangular limitation threshold appears very marked, it corresponds, forthe detector, to the transition from a rather absorbing state to arather reflecting state. For slightly different pitches a of the grating3, the angular limitation effect appears for substantially differentvalues, the angular selectivity is therefore very sensitive to the pitcha of the grating. Thus the choice of a pitch a of the grating 3 matchedto the envisaged application makes it possible to obtain the desiredlimitation of the viewing angle of the detector correspondingadvantageously to the viewing angle of the observed scene. The curve Dshows the variation of the overall reflection coefficient of thedetector where the grating 3 is absent. It is clear that the detector isthen in an absorbing state whatever the angle of incidence of theincident radiation, and that the effect of limiting the viewing angle ofthe detector is virtually absent. The grating 3 of the blocks 30 and ofpitch a less than the mean wavelength λ₀ of the spectral sensitivityrange is at the origin of this effect of limiting the viewing angle ofthe detector, which is very marked.

FIG. 4 shows schematically an example demonstrating the spectralselectivity of a thermal detector according to the invention, and itrelates to the same example of the detector as that of FIG. 3. FIG. 4shows the variation in the overall reflection coefficient R, as apercentage %, according to the incident wavelength λ, expressed inmicrometers μm, each curve corresponding to a different pitch a of thegrating 3. Two curves are shown, the curve of squares corresponding to apitch a equal to 5 μm and the curve of circles corresponding to a pitcha equal to 5.2 μm. The angle of incidence of the incident radiation hasbeen taken quite close to the limit of the detector viewing angle forthese two curves, at 5 and 15 degrees, respectively. The curves exhibitquite a flat minimum around a wavelength λ equal to 11 μm, however theselective effect is more marked at lower wavelengths λ, for example,equal to about 8 μm. A thermal detector according to the inventiontherefore exhibits a certain spectral selectivity. In certainapplications, the latter may be used to further increase the absorptionof the thermal detector at certain discrete laser wavelengthscorresponding to damaging laser lines. The protection of the thermaldetector against laser damage is thus improved.

What is claimed is:
 1. An uncooled thermal matrix detector having agiven spectral sensitivity range and being formed of pixels which arethermally insulated from each other, each pixel comprising an absorbentelement which is thermosensitive in the spectral sensitivity range,characterized in that on the face turned toward the incident radiationto be detected, of each thermosensitive absorbing element is placed abiperiodic grating of elementary blocks limiting the viewing angle ofthe detector, and in that the pitch of the grating is less than the meanwavelength of the spectral sensitivity range.
 2. The detector as claimedin claim 1, characterized in that the size of each pixel is of the orderof a few times the maximum wavelength of the spectral sensitivity range.3. The detector as claimed in claim 1, characterized in that the gratinghas a thickness which is greater than a tenth of the mean wavelength ofthe spectral sensitivity range.
 4. The detector as claimed in claim 1,characterized in that the elementary blocks of the grating are ofpyramidal shape.
 5. The detector as claimed in claim 1, characterized inthat the elementary blocks of the grating are made of an electricallyconducting material of conductivity intermediate between dielectrics andperfect conductors.
 6. The detector as claimed in claim 1, characterizedin that the thermosensitive absorbing element comprises a layer of abolometric material.
 7. The detector as claimed in claim 1,characterized in that each pixel comprises successively, in thedirection of propagation of the incident radiation to be detected, thegrating, the thermosensitive absorbing element an electrically andthermally insulating layer, a substrate reflecting in the spectralsensitivity range, and in that the electrically and thermally insulatinglayer has a thickness such that, on the one hand, at about thereflecting substrate there is an electric field node and, on the otherhand, at about the thermosensitive absorbing element there is anelectric field antinode.
 8. The detector as claimed in claim 7,characterized in that the electrically and thermally insulating layer isformed by a vacuum and in that the thermosensitive absorbing element isin the form of a microbridge connected to the reflecting substrate byelectrical connection leads the thermal conductivity of which is lessthan a given threshold.
 9. The detector as claimed in claim 7,characterized in that the sum of the thickness of the electrically andthermally insulating layer and of half of the thickness of thethermosensitive absorbing element is almost equal to a quarter of themean wavelength of the spectral sensitivity range.
 10. An opticalarchitecture comprising a detector according to claim 1, characterizedin that the optical architecture also comprises entrance opticscollecting the incident radiation to be detected and having a diameter Φand a focal length f, the viewing angle of the detector is limited to avalue θ which is almost equal to 2 arctan (Φ/2f).